The field to which our invention relates is that known as Electrical Discharge Machining, hereinafter referred to as "EDM" in which material is removed from an electrically conductive workpiece by the action of electrical gap discharges between one or more electrodes and the workpiece. In some cases, the standard polarity of the negative electrode and positive workpiece may be reversed. The servo feed system is used to provide relative movement during machining thus to maintain a relatively constant optimum spacing between the electrode and workpiece as material is progressively removed from the workpiece.
A dielectric fluid is circulated and recirculated generally under pressure to the gap throughout the machining operation. For most reliable results, a power supply of the independent pulse generator type is utilized to provide machining power pulses of precisely controllable frequency and duty factor. One type of multivibrator, particularly suitable for use in an EDM power supply is described and disclosed in Bell, U.S. Pat. No. 3,809,848, issued on May 7, 1974, for "Digitally Controlled Power Supply for Electrical Discharge Machining Apparatus". Servo control circuits employing field effect transistors and used for handling single gap setups are known from Bell, U.S. Pat. No. 3,727,024, issued on Apr. 10, 1973, for "Electrical Discharge Machining Servo Control Circuit" and from Lobur, U.S. Pat. No. 3,558,844, issued on Jan. 26, 1971, for "Electrical Discharge Machining Servo Control Circuit". Both the above-mentioned patents are of common ownership with the present application.